A word on the legal position, which is very plain. To attack a foreign flagged vessel in international waters is illegal. It is not piracy, as the Israeli vessels carried a military commission. It is rather an act of illegal warfare.
Because the incident took place on the high seas does not mean however that international law is the only applicable law. The Law of the Sea is quite plain that, when an incident takes place
on a ship on the high seas (outside anybody's territorial waters) the applicable law is that of the flag state of the ship on which the incident occurred. In legal terms, the Turkish ship was Turkish territory.
There are therefore two clear legal possibilities.
Possibility one is that the Israeli commandos were acting on behalf of the government of Israel in killing the activists on the ships. In that case Israel is in a position of war with Turkey, and the act falls under international jurisdiction as a war crime.
Possibility two is that, if the killings were not authorised Israeli military action, they were acts of murder under Turkish jurisdiction. If Israel does not consider itself in a position of war with Turkey, then it must hand over the commandos involved for trial in Turkey under Turkish law.
In brief, if Israel and Turkey are not at war, then it is Turkish law which is applicable to what happened on the ship. It is for Turkey, not Israel, to carry out any inquiry or investigation into events and to initiate any prosecutions. Israel is obliged to hand over indicted personnel for prosecution.